Locally applied high air temperature significantly altered bunch stem and grape berry physiology

Abstract

Climate change projections show major changes regarding temperature: a global warming with an increase average temperature and in frequency and severity of punctual heat events. Such changes in temperature will significantly affect the grapevine growing cycle and the bunch microclimate, known to be a major regulator of grape metabolism. A heating system adapted from Tarara et al. (2000) was built to heat individual bunches without changing their exposure. Two experiments were performed to screen the effect of several independent parameters related to temperature. The first experiment used a factorial design and compared average day temperature (25 versus 40 °C) and night temperature (20 versus 30 °C). The second one combined three intensity levels (30, 37 and 44 °C) and five durations (varying from 3 to 39 hours) of heat treatments using a Doehlert design. Experiments were conducted in a UV-transparent glasshouse, where temperature was recorded using thermocouples (air) and a thermal camera (bunch). Light, humidity, CO₂ and vine physiology were either controlled or monitored. Experiment 1 showed that day temperature had a greater effect on berry weight compared to night temperature. However, both induced a change in maturation rate and berry composition. Experiment 2 showed that the effect of temperature on berry weight was proportional to the intensity of temperature. Also, average heat at 44 °C induced bunch stem necrosis and berry shrivelling symptoms, independently of the length of the treatment, suggesting that changes in physiology were irreversible for berries reaching such a high temperature even for a short amount of time.